Optical data medium driving apparatus and a servo circuit with normalization processing

ABSTRACT

An optical data medium driving apparatus which constitutes amplifiers for an output current signal of photosensor by a plurality of current mirror circuits different in an amplification factor in order to switch in a wide range the output current signal of the photosensor which detects quantity of the reflected light corresponding to variation of the quantity of reflected light from an optical data medium having various characteristics, thereby obtaining the output with high accuracy and in a wide band, and a servo circuit which uses for an AGC circuit nomalizing a difference signal from each light receiving surface at a photosensor having a plurality of the light receiving surfaces an A/D converter which is given the difference signal as an analog input signal and the sum signal as the reference input signal and outputs an offset binary type digital signal, thereby being inexpensive to be produced, easy to be integrated and have high function, and high in reliability.

This application is a divisional application of application Ser. No.07/212,566 filed on Jun. 28, 1988 now U.S. Pat. No. 5,204,901.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical data medium driving apparatus, suchas an optical disc or a photomagnetic disc, and more particularly to anoptical data medium driving apparatus designed by paying attention to anamplifier for an output current of a detector detecting an error signalfor driving a tracking servo mechanism and a focusing servo mechanism,and relates to a servo circuit for position controlling the light spotof, for example, an optical disc apparatus, especially a servo circuitinexpensive to be produced and of high reliability.

2. Description of the Prior Art

Recently, an information processing amount by a computer has increasedsteadily and an optical disc apparatus of larger recording amount hasbeen noticed in order to record and reproduce the increased information,therefore the technology thereof is remarkably improving. In the opticaldisc apparatus, the information recorded on a data medium or to berecorded thereon is a micropit of 1 μm or less, whereby a focusing servomechanism or a tracking servo mechanism is indispensable which restrictsthe laser beam to a micro spot light of diameter of about 1 μm to allowthe laser light to impinge always on the medium surface regardless ofsurface deflection or track deflection of the data medium, wherebydevelopment of a servo mechanism of high accuracy has been desired.

In the conventional optical disc apparatus or a photomagnetic discapparatus, when the information is recorded, there is a difference inthe relative peripheral speed between an optical head and a disc at aninner periphery or an outer periphery. Hence, for obtaining theequivalent condition on the disc it is required to intensify therecording laser beam as it approaches the outer periphery. For thiscontrol, the information of track number prerecorded on the disc isdemodulated and D/A converted, thereby controlling the laser power.

Meanwhile, since the servo system for driving the optical head, such asa tracking servo or a focusing servo, utilizes the reflected ortransmitted light, a servo driving signal changes when the laser powerchanges or a reflection factor changes. Accordingly, in order to obtaina stable servo driving signal, the servo gain is required to beautomatically changed so that the averaged servo driving signal may beobtained regardless of variation in the laser power and reflectionfactor of the disc.

Next, explanation will be given on a concrete example of an amplifierwhich automatically changes the servo gain, disclosed in the JapanesePatent Application Laid-Open Gazette No. 22746/1985.

FIG. 1 shows the conventional servo gain control circuit, in whichreference numeral 117 designates a photosensor including four receivingsurfaces known in the cylindrical lens method, which outputs from theupper side shown in the drawing three kinds of systems of a track errorsignal, a focus error signal and a total quantity of reflected lightsignal. Reference numeral 118 in the track error signal systemdesignates a differential amplifier, and 119 and 121 designatesamplifiers respectively. These amplifiers 118, 119 and 121 are connectedin series and outputting a track servo driving signal to an outputterminal 122. Reference numeral 123 in the focus error signal systemdesignates a differential amplifier, and 124 and 126 designateamplifiers respectively, these amplifiers 123, 124 and 126 beingconnected in series and outputting the focus servo driving signal to anoutput terminal 127. The total quantity of reflected light system is soconstituted that an inverting amplifier 128 and a filter 129 areconnected in series, an output of the filter 129 is of negative polarityand connected in parallel to gate terminals G of field-effecttransistors (to be hereinafter called FET) 120 and 125, the sourceterminals S of the FETs 120 and 125 are grounded, and the drainterminals D are connected to one input terminals of amplifiers 119 and124 respectively.

FIG. 2 is a graph showing an example of a characteristic curve of gatevoltage V_(GS) to a resistance value R_(DS) between the equivalent drainand the source of the FETs, which has a characteristic of a curve ofsecondary degree, wherein the center point of a linear region isrepresented by P.

Next, explanation will be given on operation of the servo gain controlcircuit shown in FIG. 1.

The output of the amplifier 128 is a total quantity of reflected lightsignal, which is removed by the filter 129 of noise component and thenconnected in parallel to gate terminals G at the FETs 120 and 125respectively, so that the level variation in the total quantity ofreflected light signal is applied as gate voltage V_(GS) to theterminals G, and a resistance value R_(DS) between the equivalent drainand source corresponding to the gate voltage V_(GS) represented by thecharacteristic curve shown in FIG. 2 varies. When a resistance value ofFET 120 is represented by R_(DS4), that of FET 125 by R_(DS9),resistance between one input terminal and the output terminal at theamplifier 119 by R₃, a resistance between the same at the amplifier 124by R₈, and amplifier gains of the amplifiers 119 and 124 by A₃ and A₈,A₃ and A₈ are given in the following expressions: ##EQU1## whereby theamplifier gains are automatically controlled corresponding to variationin the resistance value R_(DS) of each FET. In other words, as the totalquantity of reflected light reduces, gate voltage V_(GS) decreases inproportion thereto. Conversely, the servo driving signal output isdesigned to be averaged by utilizing an increase in the amplifier gain.

Since an intensity ratio of the laser light is large during therecording and the reproducing of the information, when the informationis reproduced, the output of total quantity of reflected light, that is,the output of amplifier 128, is small, whereby a region of bad linearityat the FET characteristic shown in FIG. 2 is obliged to be used, therebycausing the defect that sufficient amplifier gain is not obtainable anda stable servo circuit is not realizable.

As the countermeasure for the above conventional defect, it isconsidered that during the reproduction the servo gain is made largerand during the recording the servo gain is changed corresponding to theoutput of total quantity of reflected light of an optical head, so thatan automatic adjusting circuit for servo gain obtainable of a stabledrive output for the optical head both when the information isreproduced and recorded regardless of variation in the total quantity ofreflected light, is used.

In the disc apparatus provided with amplifiers for amplifying the trackerror signal and focus error signal from the optical head so as toadjust the servo gain for operating the tracking servo and focusingservo by the outputs of respective amplifiers, it is considered as theimprovement for the above defect that a changeover switch for switchingthe gains of both the amplifiers by the switching signal for therecording and reproducing is provided and control means which stabilizesthe outputs of both the amplifiers at about the equal level in spite ofseparation of the recording from the reproducing is provided.

FIG. 3 is a structural view of an example of the automatic adjustingcircuit for the servo gain by applying the above improvement, in whichonly the components different from the conventional example shown inFIG. 1 are shown.

In FIG. 3, reference numerals 130 and 131 designate analog switcheshaving contacts 136, 137 and 138, 139 respectively, which are driven byrecording/reproducing gate signals inputted to the terminal 133, 132designates an amplifier into which the total quantity of reflected lightsignal passing a filter 129 is inputted, and is output connects to thegate terminal G of a FET 120 through a contact 138 of the analog switch131. The output of the filter 129 is branched to connect to the gateterminal G of the FET 120 through an contact 139. The source terminal Sof the FET 120 is grounded, and the drain terminal D connects with oneinput terminal of an amplifier 119 and connects in common with theoutput terminal of the amplifier 119 through contacts 136 and 137 andresistances R134 and R135 at the analog switch 130 respectively.

Next, explanation will be given on operation of the circuit shown inFIG. 3. The contacts 136 and 138 at the respective analog switches 130and 131 are closed and those 137 and 136 are open when the input signalto the terminal 13 is reproduced, and they operate reversely when theinput signal is recorded.

At first, the total quantity of reflected light signal obtained by allthe composite outputs of the photosensor 117 shown in FIG. 1 isamplified so that the output is of negative polarity by the invertingamplifier 128. When the output is inputted as gate voltage to the FET120 through the filter 129 at the next stage, the gain of amplifier 128is so set that the mean value of inputted gate voltage, when theinformation is recorded, is positioned to get a proper linearity in FIG.2 (namely the point P). When the information is reproduced, the laserpower is lowered, and the amplifier (132) gain is so set that theinputted gate voltage at that time is about equal to the mean valueduring the recording. At this state, when the recording/reproducing gatesignal is applied to the terminal 133, the input gate voltage of the FET120 during the recording vertically varies in a range of nearly properlinearity around the point P on the characteristic curve in FIG. 2 andthe gate voltage during the reproduction does not so fluctuate from theoutput (gate voltage access to the operating point P) decided by thefunction limit of amplifier 132.

Next, the servo gain is set. For example, when the track error signal ofphotosensor 117 is amplified by the differential amplifier 118, itsoutput represents the direction and extent of track error, thereby beingconnected to a coil moving an objective lens to correct the track errorthrough the amplifiers 119 and 121. Herein, the servo gain is set by theamplifiers 118 and 119, but the amplifier (119) gain is decided by theresistance value R_(DS4) between the equivalent drain and source of FET120 and the resistances R134 and R135. When the amplifier (119) gain isrepresented by A_(3W) when recorded and by A_(3R) when reproduced, thegain is given in the following expressions: ##EQU2##

Here, when the gain of amplifier 128 is set on a basis of the mean valueof the laser power during the recording, since the laser power duringthe reproducing is low, even when the gain of amplifier 132 is set, theservo gain tends to be short. Hence, the analog switch 130 is controlledby the recording/reproducing gate and values of resistance R134 and R135are selected so as to obtain driving voltage to correct the track errorsto about an equal extent during the recording and reproducing, therebyenabling the most suitable servo gain to be set.

Now, the circuit shown in FIG. 3 is described as to the track servodriving signal system, the focus error signal system is applicable inthe same way as in FIG. 3. In the FIG. 3 circuit, the differentialamplifier 118 is replaced by that 123 and the amplifiers 119 and 121 bythose 124 and 126, thereby enabling a stable focus servo driving signalto be obtained.

As above-mentioned, the improved automatic gain control circuit for theservo gain is simple in construction, and, even when the output of thetotal quantity of reflected light fluctuates regardless division of theinformation for recording/reproducing, can always easily obtain thestable servo driving signal.

The conventional optical data medium driving apparatus and animprovement thereof are constructed as above-mentioned. In other words,the apparatus is provided with function (to be hereinafter called theauto gain control: AGC) such that the servo driving signal is divided bythe total quantity of reflected light to average the servo drivingsignal to thereby maintain constant the servo loop gain regardless ofvariation in the laser power and reflection factor of the disc. Also, inorder to compensate a narrow dynamic range of AGC, the gain switchingstage for switching the gain at the step of reproducing/recording isprovided at the preceding stage of the AGC.

In consideration of accuracy (about ±1 μm for focusing servo and about±1 μm for tracking servo) and band (about 3 KHz for the gain crossoverfrequency) required to the optical disc servo, as the characteristic ofAGC, the dynamic range is limited to five times through ten times theextent.

However, there are some data mediums for the optical disc having variouscharacteristics. In consideration of the reflection factor afterrecording inclusive, variation in the reflection factor of the opticaldisc becomes about ten times larger.

The reproduction power is required to be changed to meet with thesensitivity of data medium and not to break the recording data. Also,the recording power is different depending on the kind of data medium.Furthermore, in consideration of an apparatus, such as the photomagneticdisc, which continuously lights the semiconductor laser by the powerequivalent to or larger than the recording power to thereby erasing therecording data, even average variation in the laser power becomes aboutten times larger.

Accordingly, for the optical disc or the photomagnetic disc, as arecording medium, having various characteristics mentioned above, theproblem is created in that the conventional example only enlarging theAGC dynamic range by switching the preceding reproducing/recording isinsufficient.

Meanwhile, there is another prior art relating to the AGC.

FIG. 4 is a block diagram of the servo circuit of the conventionaloptical data medium apparatus disclosed in the Japanese PatentPublication Gazette No. 28653/1983.

In FIG. 4, reference numeral 201 designates a photosensor for detectinga focus error of the light spot (not shown), which comprises two-dividedlight receiving surfaces 201a and 201b, reference numerals 202 and 203designate current-voltage converters (to be hereinafter referred to asIV converter) which convert photocurrent signals inputted from therespective light receiving surfaces 201a and 201b into voltage.Reference numerals 204 and 205 designate operational amplifiers forobtaining a difference signal V_(X) and a sum signal V_(Y) of eachoutput of the respective IV converters 202 and 203. Reference numeral206 designates an analog divider which divides the difference signalV_(X) by the sum signal V_(Y) and constitutes normalization processingmeans for normalizing by the sum signal V_(Y) the level variation ofdifference signal V_(X) to be constant. Reference numeral 207 designatesan actuator driving circuit for amplifying the output of analog divider206. Reference numeral 208 designates a focus actuator for controllingthe focal position of the light spot. The above components 201 through208 comprise the auto focusing servo system.

Similarly, the auto tracking servo system comprises a photo sensor 209comprising the two-divided light receiving surfaces 209a and 209b fordetecting the track error of the light spot, IV converter 210 and 211,an operational amplifier for obtaining the difference signal V_(Z) ofthe photosensor 209, analog divider 213, actuator driving circuit 214,and tracking actuator 215 for controlling tracking of the light spot. Inaddition, the sum signal V_(Y) inputted to the analog divider 213 at theauto-tracking servo system uses the sum signal V_(Y) of the divisionamplifier 205 at the auto focusing servo system.

Next, explanation will be given on operation of the conventional servocircuit shown in FIG. 4.

At first, the light receiving surfaces 201a and 201b at the focus errorsensor 201 output a photocurrent corresponding to a received lightquantity, the IV converters 202 and 203 converting current signals fromthe light receiving surfaces 201a and 201b into voltage.

The operational amplifiers 204 and 205 compute the difference signalV_(X) and sum signal V_(Y) of voltages from the respective IV converters202 and 203, the analog divider 206 divides the difference signal V_(X)by the sum signal V_(Y). The actuator driving circuit 207 amplifiers theoutput of the analog divider 206 to drive the focus actuator 208 topositioning-control the light spot to be constant.

Similarly, the actuator driving circuit 214 drives the tracking actuator215 corresponding to imbalance in quantity of received light on thelight receiving surfaces 209a and 209b at the photosensor 209 so thatthe irradiation position of the light spot is positioning-controlled ona desired track.

Generally, when variation in the output from the light source for thelight spot changes in phase the quantity of received light of thephotosensors 201 and 209, the output level of difference signal V_(X)with respect to the equal control error amount varies. The aforesaidanalog divider 206 is used to normalize variation in output levelcorresponding to the quantity of received light, whereby the quantity ofcontrol error of the stable servo loop is obtained.

Such normalization processing means for dividing the difference signalV_(X) by the sum signal V_(Y) is well-known as disclosed in, forexample, the Japanese Patent Publication Gazette 5293/1980.

The conventional servo circuit, as above-mentioned, uses the analogdivider 206 in order to normalize the difference signal V_(X). Since theanalog divider 206 is required to have the frequency characteristic tothe extend that a phase lag at the target servo region is notproblematical and the dynamic range and accuracy in the range that anoffset to cause a control error in the servo loop is not problematical,there is the problem that an expensive IC applied with laser trimmingmay inevitably be used.

Since the signal normalized by the analog divider 206 is an analogsignal, drift or aging cannot be neglected and the reliability isdifficult to be maintained, thereby creating the problem in that allcircuits must be analog processed and high analog technology arerequired to add function to have high performance.

SUMMARY OF THE INVENTION

The present invention has been designed in order to solve the aboveproblem. A first object of the invention is to provide an optical datamedium driving apparatus which uses a required number of current mirrorcircuits at an amplifier for amplifying an output current of aphotosensor for obtaining a required servo driving signal, and isprovided with gain switching means which computes a required current asto the amplified output current so as to compensate a dynamic range ofAGC with respect to an optical disc or a photo magnetic disc as datamedium having various characteristics, and is provided with an amplifierobtainable of a servo driving signal of high accuracy and a wide band.

A second object of the invention is to provide a servo circuitinexpensive to be produced, ease to integrate and have high function,and high in reliability, by providing an AD converter as normalizationprocessing means outputting an offset binary type digital signal usingthe difference signal as the analog input and the sum signal as thereference input.

The above and further objects and features of the invention will morefully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the conventional servo gain controlcircuit,

FIG. 2 is an illustration of operation thereof,

FIG. 3 is a circuit diagram of an improved conventional example of theservo gain control circuit,

FIG. 4 is a block diagram of another conventional example of the servocircuit,

FIG. 5 is a circuit diagram of a photosensor and amplification means ofa first embodiment of an optical data medium driving circuit of a firstinvention,

FIGS 6a-6c is a circuit diagram of a photosensor and amplification meansof a second embodiment of the first invention,

FIG. 6b is a table illustrating I_(OUT) as a function of switchpositions in the circuit shown in FIG. 6a,

FIG. 7 is a circuit diagram of a photosensor and amplification means ofa third embodiment of the first invention,

FIG. 8 is a circuit diagram of a photosensor and amplification means ofa fourth embodiment of the first embodiment,

FIG. 9 is a circuit diagram of a photosensor, a current-voltageconverter circuit, a voltage-current converter circuit, andamplification means, of a fifth embodiment of the first invention,

FIG. 10 is a circuit diagram of a photosensor, amplification means, anda post computer, of a sixth embodiment of the first invention,

FIG. 11 is a circuit diagram of a current mirror circuit in detail atthe sixth embodiment of the first invention,

FIG. 12 is a block diagram of a first embodiment of normalizationprocessing means of a second invention,

FIG. 13 is a block diagram of a second embodiment of normalizationprocessing means of the second invention, and

FIG. 14 is a block diagram of a servo circuit of the second invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 5 is a circuit diagram of a photosensor and amplification means ofa first embodiment of an optical data medium driving apparatus of thefirst invention.

In FIG. 5, an output current I_(PD) of a photosensor 1 is amplified by acurrent mirror circuit 15 comprising transistors 2 through 8 of arrangedcharacteristics, and resistances 9 through 14, thereby enabling thepredetermined-fold (for example, 5-fold) output current I_(out) to beobtained.

Next, explanation will be given on operation of the optical data mediumapparatus shown in FIG. 5.

When the light impinges to the photosensor 1 comprising a PINphotodiode, the output current I_(PD) in proportion to energy of theimpinging light is obtained. Assuming that voltage between the emitterand the base of each transistor 2 or 3 is V_(BE) and a resistance valueof resistance 9 is R₉, the reversed bias voltage E_(PD) needful to thePIN photodiode is given in the following equation:

    E.sub.PD ≈V.sub.CC -2V.sub.BE -I.sub.PD.R.sub.9.   (1)

It is recognized from the equation (1) that R₉ corresponding to themaximum value of I_(PD) is selected to obtain a sufficiently largerE_(PD) without sacrificing is frequency characteristic. In other words,for example, by selecting I_(PD) ·R₉ to be about equal to V_(BE),

E_(PD) =V_(CC) -3V_(BE) =V_(CC) -2.1V and the reverse bias voltage nearthe supply voltage are applicable.

As seen from FIG. 5, since the base potential of the respectivetransistors 3 through 8 is always identical with each other, voltagesV_(BE) 3 through V_(BE) 8 between the emitter and the base at thetransistors 3 through 8 and resistance values R₉ through R₁₄ ofresistances 9 through 14 corresponding to the above are adjustedcorresponding to the required accuracy of the amplifier, whereby therelation between the emitter currents I₉ through I₁₄ flowing inresistances 9 through 14 is given in the following equation. ##EQU3##

The following relation should hold in a range of desired accuracy:

    I.sub.9 =I.sub.10 =. . . I.sub.14                          (3)

Such operation is easily realizable by integrating close to each otheron the same wafer the transistors 3 through 8 and resistances 9 through14 corresponding thereto.

On the other hand, the collector current of transistor 3 is a differenceobtained by subtracting the base current with respect to the transistor2 from the output current I_(PD) of the photosensor 1. The base currentwith respect to the transistor 2, when hfe of the respective transistors2 through 8 is assumed to be equal to each other, is given in thefollowing equation: ##EQU4##

Assuming that hfe is enlarged to be, for example, 78 or more as to anytransistor, it is recognized from the equation (4) that the collectorcurrent of transistor 3 coincides with the output current I_(PD) of thephotosensor 1 with accuracy of 0.1% or less.

Accordingly, the emitter current I₉ of transistor 3 is the sum of itsbase current and collector current, whereby I₉ in a range of accuracydecided by hfe of the transistor is given in the following expression:##EQU5##

Also, the relations among the emitter currents I₁₀ to I₁₄ and thecollector currents I₄ to I₈ at the transistors 4 to 8 are given in thefollowing expressions: ##EQU6##

As seen from the expression (3), since the emitter currents I₉ of I₁₄ atthe transistors 3 to 8 are equal to each other, when hfe of therespective transistors is in a predetermined range, the base currentswith respect to the transistors 3 to 8 are included in a predeterminedrange, and following expression is obtained from the expressions (3),(5) and (6):

    I.sub.PD =I.sub.4 =I.sub.5 =. . . =I.sub.8.                (7)

As seen from the expression (7), the output currents I₄ to I₈ equal tothe input current I_(PD) appear as it is reflected by the mirror,whereby the circuit 15 comprising transistors 2 to 8 and resistances 9to 14 corresponding thereto is called the current mirror circuit.

The output current I_(OUT) of the current mirror circuit 15 given as thesum of collector currents I₄ through I₈ of transistors 4 through 8 inthe following expression:

    I.sub.OUT =I.sub.4 +I.sub.5 +. . . +I.sub.8 =5.I.sub.PD.   (8)

Thus, it is recognized from the expression (8) that the current mirrorcircuit 15 operates as an amplifier having a 5-fold currentamplification factor.

Such current mirror circuit, especially when integrated, can have highaccuracy with ease.

FIG. 6a is a circuit diagram of a photosensor and amplifiers of a secondembodiment of an optical data medium drawing apparatus of the firstinvention.

In FIG. 6a, the output current I_(PD) of the photosensor 1 is amplifiedstepwise (for example, one-fold, two-fold or four-fold) by a currentmirror circuit 36 comprising transistors 2, 3 and 16 to 22 arrangingcharacteristics thereof and resistances 9 and 23 to 29. Such stepwiseamplified current is switched by switches 37, 38 and 39 comprisingtransistors 30 through 35, thereby obtaining an output current I_(OUT)of the properly amplified I_(PD).

Next, explanation will be given on operation of the above.

The photosensor 1 and current mirror circuit 36 operate in the same wayas those in FIG. 5, whereby the collector currents I₁₆ to I₂₂ oftransistors 16 to 22 are equal to I_(PD).

Meanwhile, the transistors 30 and 31, 32 and 33, and 34 and 35constitute switches respectively, so that the current amplified by thecurrent mirror circuit 36 is switched toward V_(CC) side or the outputterminal side. For example, at a switch 37 comprising the transistors 30and 31, when the base potential of transistor 30>base potential oftransistor

    31+0.7 V                                                   (9),

the transistor 30 is ON between the base and the emitter and that 31 isOFF between the base and the emitter, whereby the collector current I₁₆of transistor 16 is coupled with V_(CC) side through distance betweenthe collector and the emitter. On the contrary, when

the base potential of transistor 31>the base potential of transistor

    30+0.7 V                                                   (9)',

the collector current I₁₆ of transistor 16 is outputted through distancebetween the collector and the emitter of transistor 31.

Here, assuming that when the transistors 30, 32 and 34 at the V_(CC)side of the switches 37, 38 and 39 are ON, these switches go to logical"0". On the contrary, when the transistors 31, 33 and 35 at the outputside are ON, they go to logical "1", as shown in FIG. 6b, I_(OUT) ischanged stepwise from 0 to I₁₆ +I₁₇ +I₁₈ +I₁₉ +I₂₀ +I₂₁ +I₂₂.

Since the following condition:

    I.sub.PD =I.sub.16 =I.sub.17 =I.sub.18 =. . . =I.sub.22.   (10)

should hold, I_(OUT) varies from 0 to 1·I_(PD) , 2·I_(PD), . . . ,7·I_(PD).

Thus, the current mirror circuit 36 and switches 37, 38 and 39 operateas an amplifier depending on conditions of the switches 37, 38 and 39and having current amplification factors of eight stages from 0-fold to7-fold stages.

In addition, although the accuracy is problematical, the current mirrorcircuit 36 may be constituted as discussed below with respect to FIGS. 7and 8.

FIG. 7 is a circuit diagram of a third embodiment of the optical datamedium driving apparatus of the first embodiment.

Referring to FIG. 7, two transistors 17 and 18 in FIG. 6 are replaced byone transistor 17, and similarly four transistors 19, 20, 21 and 22 byone transistor 19. In FIG. 7, the collector currents flowing in thetransistors 16, 17 and 19 are different from each other, so that whenthe characteristics of transistors are uniform, voltage between theemitter and the base of each transistor is different from each other,thereby deteriorating accuracy of the current mirror circuit to thatextent. Accordingly, when such a current mirror circuit as shown in FIG.7 is adopted, the emitter areas of transistors 17 and 19 are madeseveral times larger than that of transistor 16 and voltage between theemitter and the base of each transistor is arranged, thereby preventingthe aforesaid accuracy from deteriorating to a certain extent.

FIG. 8 is a circuit diagram of a fourth embodiment of the firstinvention, in which a current mirror circuit 36 has the emitter areas oftransistors 40 and 41 two times and from times larger respectively thanthat of transistor 16, and various resistances are not shown in FIG. 8.

Assuming that hfe of the respective transistors 3, 16, 40 and 41 can bearranged with needful accuracy, since the base potential of transistors3, 16, 40 and 41 is all equal to each other, the emitter currents oftransistors 3 and 16 are equal to each other, the emitter current oftransistor 40 is two times larger than that transistor 16, and theemitter current of transistor 41 is four times larger than that oftransistor 16. The corresponding base currents similarly become twotimes and four times larger.

Hence, the collector current of transistor 16 is equal to an inputcurrent I_(PD) to the current mirror circuit 36, that of transistor 40is equal to 2.I_(PD), and that of transistor 41 is equal to 4.I_(PD).

In addition, referring to FIGS. 6a, 6b 7 and 8, the currentamplification factors of the current mirror circuit at the plural stagesare selected to be 2^(R) (R: integer, and the values for the above: 0, 1and 2) and the values are switched by a proper current switch so thatthe entire gain is changed at 2^(m) stages (m: the number of stages inthe current mirror circuit, m=3 for the above). Alternatively, the gainmay be nonlinearly changed by changing weight so as to widen the dynamicrange by the current mirror circuit of the stages less in number.

FIG. 9 is a circuit diagram of a fifth embodiment of the optical datamedium driving apparatus of the first invention, mainly showing aphotosensor, a current-voltage conversion circuit, a voltage-currentconversion circuit and amplifiers.

In FIG. 9, an output current of a photosensor 1 is converted into thecorresponding voltage signal by a current-voltage conversion circuit 45comprising a resistance 43, an operational amplifier 42 and a powersource 44. The voltage signal is transmitted to a transmission line 46and then reconverted into the corresponding current signal byvoltage-current conversion circuit 51 comprising an operationalamplifier 47, a transistor 48, and resistances 49 and 50. The currentsignal thus converted is applied to a constant current circuit 58comprising the transistors 52 through 54 and the resistances 55 through57. The collector currents outputted from the transistors 52 through 54are equal in intensity to the reconverted current signal due to theproperty as the aforesaid current mirror circuit.

The collector current thus outputted is switched by switches 63 and 64comprising transistors 59 through 62 respectively, so that the outputcurrent I_(OUT) 0-fold, 1-fold, 2-fold or 3-fold as much as I_(PD)(wherein resistance values of resistances 43 and 50 are made equal toeach other) may be selectively obtained.

Next, explanation will be given on operation of the above.

The output current I_(PD) of the photosensor 1 flows in the resistance43 and potential at the point a is equalized to voltage E₀ of powersource 44 due to operation of the operational amplifier 42. Now,assuming that a resistance value of resistance 43 is R₄₃, the output ofoperational amplifier 42 is a voltage signal of E₀ -I_(PD) ·R₄₃. Thevoltage signal and supply voltage E₀ are transmitted to the transmissionline comprising, for example, a two-core shielded cable and thenpotential at the point b is made to be E₀ -I_(PD) ·R₄₃ by the operationof the optical amplifier 47. When the resistance value of resistance 50is represented by R₅₀, the output is converted into a current signal asfollows: ##EQU7##

In addition, when R₄₃ =R₅₀, the initial current I_(PD) is restored.

The I_(PD) becomes the collector current of transistor 48 and flowstogether with its base current in the resistance 49. As shown in FIG. 9,the base potential at the transistors 48 and 52 through 54 is alwayscoincident, so that characteristics of the transistors and theresistances 49 and 55 through 57 are arranged to carry out the aforesaidfunction as the current mirror circuit. Therefore, the collectorcurrents of transistors 52 through 54 coincide, with a predeterminedaccuracy, so that the constant current circuit 58 comprising thetransistors 52 through 54 and resistances 55 through 57 operates in thesame way as the current mirror circuit. Thus, the constant currentcircuit 58 outputs a current I₁, and a current I₂, 1-fold and 2-fold asmuch as I_(PD) respectively. Hence, the gain can be changed across fourstages from the 0-fold to 3-fold value depending upon the switchingoperation by the switches 63 and 64 comprising the transistors 59through 62 and the input code with respect to said switches.

FIG. 10 is a circuit diagram of a sixth embodiment of the optical datamedium driving apparatus of the first invention, mainly showingphotosensors, amplifiers and a post current computer.

FIG. 10 shows the case where output currents I_(PD) (a) and I_(PD) (b)of two photosensors 1a and 1b are amplified corresponding to apredetermined input code (SW input) by current amplifiers 36a and 36b.

From the amplified currents I_(OUT) (a) and I_(OUT) (b) are generated+I_(OUT) (a), +I_(OUT) (a), -I_(OUT) (b) and +I_(OUT) (b) by a currentmirror circuit 86 comprising transistors 66 and 67 and resistances 68and 69, a current mirror circuit 87 comprising transistors 70, 71 and 72and resistances 73, 74 and 75, a current mirror 88 comprisingtransistors 76, 77 and 78 and resistances 79, 80 and 81, and a currentmirror circuit 89 comprising transistors 82 and 83 and resistances 84and 85, these currents flowing in resistances 90 and 91 leading to carryout a required current computation. As the result of the computation,V_(DEF) and V_(ADD) are given in the following equations:

    V.sub.DEF =V.sub.ref -{I.sub.OUT (a)-I.sub.OUT (b)}·R.sub.90

    V.sub.ADD =V.sub.ref -{I.sub.OUT (a)+I.sub.OUT (b)}·R.sub.91,

where resistance values of resistances 90 and 91 are R₉₀ and R₉₁.

Next, explanation will be given on operation of the above.

The internal operation of the respective current mirror circuits 86, 87,88 and 89 are the same as the above-mentioned, description of which isomitted. An output current I_(PD) (a) of the photosensor 1a is amplifiedby the current amplifier 36a to be I_(OUT) (a). The amplified outputcurrent at last is equal to the collector currents of transistors 71 and72 by operations of the current mirror circuits 86 and 87. On the otherhand, an output current I_(PD) (b) of the photosensor 1b is amplified bythe current amplifier 36b to be I_(out) (b). The amplified outputcurrent is operated by the current mirror circuits 88 and 89 to lastlybe equal in the collector currents of transistors 78 and 83. Thetransistor 71 is of the NPN type and that 78 is of the PNP type, wherebythe collector currents of both the transistors 71 and 78 are differentin the direction from each other. Accordingly, when these collectorcurrents are added so that the circuit is wired to flow the currents inthe resistance 90, the added collector current is converted into avoltage signal as a voltage drop from the V_(ref) potential. Theconverted output V_(DEF) is given in the following equation:

    V.sub.DEF =V.sub.ref -{I.sub.out (a)-I.sub.out (b)}·R.sub.90

thus obtaining a difference signal between I_(out) (a) and I_(out) (b).The transistors 72 and 83 are both of the NPN type and the collectorcurrents of both the transistors are the same in the direction, so thatwhen the collector currents are added so that the circuit is wired toflow the currents in the resistance 91, the added collector current isconverted into the voltage signal as a voltage drop from the V_(ref)potential. The converted output V_(ADD) is given in the followingequation:

    V.sub.ADD =V.sub.ref -{I.sub.OUT (a)+I.sub.OUT (b)}·R.sub.91,

thus obtaining the sum signal of I_(OUT) (a) and I_(OUT) (b).

The difference signal thus obtained, when the photosensors 1a and 1bserve to detect the tracking error signal, is used as the tracking errorsignal for driving the tracking servo mechanism, and when the sensors 1aand 1b serve to detect the focusing error signal, is used as thefocusing error signal for driving the focusing servo mechanism.

Meanwhile, the sum signal, when the photosensors 1a and 1b are fordetecting the tracking error signal, similarly is used as the controlsignal for automatic gain control (AGC) of the tracking servo loop gain,and when for detecting the focusing error signal, is used as the controlsignal for automatic gain control (AGC) of the focusing servo loop gain.

In addition, in FIG. 10, the current mirror circuits 86, 87, 88 and 89are shown of principal structure, but in consideration of thecharacteristic of transistor to be used and the required accuracy andresponse characteristic of the circuit itself, the circuit of structureas shown in FIG. 11 is actually used.

In FIG. 11, transistors 92, 93 and 94 included in a current mirrorcircuit 86 are used for operation as the current mirror circuit at highspeed. Hence voltage between the collector and the emitter of eachtransistor 92 or 93 is used so that the collector potential oftransistors 66 and 67 constituting the current mirror circuit is notchanged by variation in output voltage of an amplifier 36a as thecurrent source. In this case, the collector potential is about equal tothe base potential. Thus, the transistor lower in withstanding voltagebut at high speed is usable as a transistor constituting the currentmirror circuit, and in the current mirror circuit, impedance of thecollector at the current mirror transistor of the highest impedance canbe lowered, thereby enabling the circuit to be operated at high speed.Similarly, transistors 97, 98, 99 and 100 included in a current mirrorcircuit 87 serve to operate at high speed as the current mirror circuitwith respect to the transistors 70, 71 and 72 constituting the currentmirror circuit, transistors 104, 105, 106 and 107 included in a currentmirror circuit 88 serve similarly with respect to transistors 76, 77 and78 constituting the current mirror circuit, and transistors 111, 112 and113 included in a current mirror circuit 89 serve similarly with respectto transistors 82 and 83 constituting the current mirror circuit.

In the current mirror circuit 86, the transistor 116 and resistance 95are for operation as the current mirror circuit at high speed and serveto lower impedance at the bases of transistors 66 and 67 constitutingthe current mirror circuit. Similarly, the transistors 102 andresistance 101 in the current mirror circuit 87 serve to operate thecurrent mirror circuit at high speed with respect to the transistors 70,71 and 72 constituting the current mirror circuit, the transistors 109and resistance 108 in the current mirror circuit 88 serve similarly withrespect to the transistors 79, 80 and 81 constituting the current mirrorcircuit, and the transistor 115 and resistance 114 in the current mirrorcircuit 89 serve similarly with respect to the transistors 82 and 83constituting the current mirror circuit.

Also, a transistor 96 included in the current mirror circuit 86 servesto raise the accuracy as the current mirror circuit. An input current tothe current mirror circuit 86 is divided into the collector current oftransistor 66 and the base current of the respective transistors 66, 67,92 and 96. When hfe of these transistors is small and intensity of thebase current with respect to the collector current cannot be neglected,the current mirror circuit deteriorate in accuracy. Hence, thetransistor 96 is interposed between the base of each transistor 66, 67,92 and 93 and the input current terminal and further a ratio of the basecurrent to the collector current is made hfe², thereby improving theaccuracy as the current mirror circuit. Similarly, a transistor 103 isprovided to improve the accuracy with respect to the current mirrorcircuit 87, a transistor 110 similarly with respect to me current mirrorcircuit 88, and a transistor 116 similarly with respect to the currentmirror circuit 89.

In addition, although explanation was not made in the first throughsixth embodiments, the data medium requiring the tracking servo andfocusing servo is not limited to the optical disc or a photo-magneticdisc, but includes an optical card, a photo sheet or an optical drum.

The data medium itself for use may be of various structures, such asmetal, oxide, various inorganic compounds, or organic compounds.

Also, the converging lens included in the apparatus may optionally use adesired lens, such as an aspherical lens, a hologram lens, andiffraction granting, or a Fresnel lens as well as the spherical lens,when having the converging lens function.

Furthermore, the photosensor for use may not be limited to the PINphotodiode, but may be of any type, such as a PN photodiode, anavalanche photodiode, PSD or a solar cell, when a signal is obtained asthe current information.

As above-mentioned, the optical data medium driving apparatus of thefirst invention, which is adapted to amplify by the required currentmirror the output current of the photosensor for detecting an errorsignal required to drive the tracking servo mechanism or the focusingservo mechanism, and can devise a wide band, low offset and low noise ofthe signal processing system, thereby having the effect that reliabilityof the entire optical data medium driving apparatus can be improved by asmall number of parts and the apparatus is small-sized and integrated.

Next, explanation will be given on a first embodiment of a secondinvention. FIG. 14 is a block diagram thereof, in which the componentsdesignated by reference numerals 201 through 205, 207 through 212, 214,and 215 are the same as those in the second conventional example.

Reference numeral 216 designates a switch for switching the respectivedifference signals V_(X) and V_(Z) from the operational amplifiers 204and 212 by time shearing.

Reference numeral 217 designates an AD converter given a differencesignal V_(X) (or V_(Z)) as an analog input and a signal V_(Y) as thereference input and outputting an offset binary type digital signal DB,which is adapted to normalize the analog difference signal V_(X) (orV_(Z)) by the sum signal V_(Y).

Reference numeral 218 designates a latch circuit comprising a flip-flopfor latching the digital signal DB, and 219 and 220 designate DAconverters which convert the digital signal DB passing through the latchcircuit 218 into analog signals A_(X) and A_(Z) corresponding to thedifference signals V_(X) and V_(Z) respectively, the analog signalsA_(X) and A_(Z) from the respective converters 219 and 220 beinginputted into the actuator driving circuits 207 and 214 respectively.

In addition, in this embodiment, the switch 216 switches the differencesignals V_(X) and V_(Z) so as to be an analog input to AD converter 217and the latch circuit 218 continuously inputs the digital signal DB tothe DA converters 219 and 220, but when structure of the same as the ADconverter is connected to the operational amplifier of tracking servosystem, the switch 216 and latch circuit 218 are needless.

Next, explanation will be given on operation of the FIG. 14 embodimentof the second invention.

At first, as is illustrated the case where the switch 216 selects thedifference signal V_(X) representing the focus error signal as shown,will be described. The AD converter 217 divides the difference signalV_(X) by the sum signal V_(Y) to execute normalization of differencesignal V_(X) so as to output the offset binary type digital signal DB.For example, when the AD converter 217 is of 8-bits, the digital signalDB obtains values of 0 to 255 dispersed corresponding to the differencesignal V_(X), but the center of the operating point is of a value in thevicinity of the code data 127. In addition, the AD converter 217 usedfor this kind of servo circuit is sufficient accurately with IC of 8-bitclass.

The digital signal DB normalized by the AD converter 217 is latched bythe latch circuit 218 and then converted into the analog signal by theDA converter 219, thereby driving a focus actuator 208 through theactuator driving circuit 207.

On the other hand, when the switch 216 is turned to input the differencesignal V_(Z) into the AD converter 217, the tracking actuator 214 isdriven on the basis of the standardized digital signal DB similarly tothe above.

FIG. 12 is a block diagram of concrete structure of an embodiment ofnormalization processing means shown in FIG. 14, in which referencenumeral 217A designates and AD converter corresponding to the ADconverter 217, and 221 designates an inverting amplifier to invert thepolarity of the sum signal V.

The AD converter 217A has to reference input terminals REF(+) and REF(-)other than an an analog input terminal IN, the difference signal V (orV) is applied as the analog input to the analog input terminal IN, thesum signal V is applied as the positive reference input to the referenceinput terminal REF(+), and a signal V-_(Y) inverted of the polarity ofthe sum signal V is applied as the negative reference input to thereference input terminal REF(-).

Next, the operation of one embodiment of normalization processing meansshown in FIG. 12 will be described with reference to FIG. 14, wherein inthe same way as the above the switch 216 is assumed to select thedifference signal V and explanation will be given paying attention tothe focus servo system.

At first, assuming that the outputs of IV converters 202 and 203 arepositive, the output offset reference values of IV converters 202 and203 are preset at V₀ (>0). However, when an electric circuit systemoperates by two positive and negative power sources, the output offsetreference value is usually set to the ground.

Since the difference signals V_(X) and V_(Z) outputted from therespective operational amplifiers 204 and 205 is superposed thereon withoffset voltage V₀ equal to the output offset reference value V₀, avoltage value of difference signal V_(X) is represented by (x+V₀), avoltage value of sum signal V_(Y) by (y+V₀), and a voltage value of asignal V-_(Y) inverted of the polarity of the sum signal V_(Y) by(-y+V₀).

The AD converter 217A compares the difference signal V and referencevoltage value from (y+V₀) to (-y+V₀) with an indented widthcorresponding to the resolution (bit number) of AD converter 217A itselfand encodes the difference signal V_(X). Accordingly, the digital signalDB, when x=y, includes the full code; when x=-y, the zero code; and,when x=0, the 1/2 full code. In detail, the digital signal DB, when theentire light spots are irradiated onto one photosensor 201a and thequantity of light received by the photosensor 201b is zero (x=y),becomes full codes. Conversely, when the quantity of received light byphotosensor 201a is zero (x=-y), the signal DB becomes zero code,thereby obtaining coded data normalized of voltage value x in thedifference signal V_(X) by the voltage value y in the sum signal V_(Y).

At this time, when the difference signal V is V₀, that is, x=0, sincethe coded data in the digital signal DB has an offset of 1/2 full code,the digital signal DB becomes the offset binary type signal.

For example, when the AD converter 217A is of 8-bits, the center ofoperating point by the 1/2 offset is a coded data value about 127 withrespect to full code 255. In other words, the point where the light spotis uniformly irradiated on each photosensor 201a or 201b, becomes thecenter of operating point and coincides with the control center of thefocus servo.

The DA converter 219 allows the zero data "0" of digital signal DB tocorrespond to -128 and full data "255" to 127 on the basis of the offsetbinary type digital signal DB thus obtained, so that an analog signalA_(x) is outputted to put the control (operating point) center to zero,thereby driving-controling the focus actuator 208.

For tracking position control, an analog signal A_(z) similarlydriving-controls a tracking actuator 215.

FIG. 13 is a block diagram of another embodiment of the normalizationprocessing means of the second invention, in which reference numeral217B designates and AD converter corresponding to the AD converter 217,231 designates an operational amplifier to obtain a half of sum signalV_(Y), and 232 designates an operational amplifier for adding a signalV_(Y/2) from the operational amplifier 231 with the difference signalV_(X).

The AD converter 217B has another reference input terminal REF otherthan the analog input terminal IN, a signal V_(X) ' (or V_(Z) ') fromthe operational amplifier 232 is inputted to the analog input terminalIN, and the sum signal V_(Y) is inputted to the reference input terminalREF.

Next, explanation will be given on operation of the normalizationprocessing means shown in FIG. 13.

The sum signal V_(Y) is a half signal V_(Y/2) by the operationalamplifier 231 and then added with the difference signal V_(X) by theoperational amplifier 232 to be the analog input signal V_(X) ', therebybeing input to the analog input terminal IN of AD converter 217B, avoltage value of the analog input signal V_(X) ' being represented by[x+(y/2)+V₀ ] because the offset voltage V₀ is superposed.

On the other hand, the sum signal V_(Y) of voltage value (y+V₀) is inputdirectly to the reference input terminal REF of AD converter 27B as thereference voltage.

The AD converter 217B compares at indented widths corresponding to theresolution of AD converter itself the voltage value [x+(y/2)+V_(o) ] ofthe analog input signal V_(X) ' with the voltage from offset voltage V₀to reference voltage (y+V₀), thereby encoding the analog input signalV_(X) '.

Accordingly, a digital signal DB' outputted from the AD converter 217Bincludes coded data which is full code when x= y/2, zero code whenx=-y/2, and 1/2 full code when x=0, resulting in that the coded data(error signal) normalizing the voltage value x by that y is included inthe same way as above-mentioned. Also, the digital signal DB' issuperposed with offset which is 1/2 full code when x=0, thereby being ofthe offset binary type signal.

Next, the actuators 208 and 215, in the same way as above-mentioned, ispositioned and driven so that the focal point of the light spot is atthe center of the servo (the operating point).

As seen from the above, when the AD converter 217 normalizing anddigitizing the error signal and outputting the offset binary typedigital signal DB is used as the normalization processing means, the ICof 8-bit class inexpensive to be produced and easy to be integrated canbe used, thereby obtaining the servo circuit easy to add function andless affected by drift and aging.

In addition, the two embodiments of the normalization means aredescribed of the optical disc apparatus as the target, but they may ofcourse be applicable to normalization of error signals at the servocircuit at the photo magnetic disc or other apparatus.

As seen from the above, the second invention uses as the normalizationprocessing means the AD converter which is given the difference signalof the error sensor as the analog input and the sum signal as thereference input so as to output the offset binary type digital signal,thereby having the effect that the servo circuit inexpensive to produce,easy to integrate and have high function, and higher in reliability.

As this invention may be embodied in several forms without departingfrom the spirit of essential characteristics thereof, the presentembodiment is therefore illustrative and not restrictive, since thescope of the invention is defined by the appended claims rather than bythe description preceding them, and all changes that fall within themeets and bounds of the claims, or equivalence of such meets and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. An optical data medium driving apparatus and aservo circuit comprising;a photosensor having a plurality of lightreceiving surfaces, said photosensor having an output corresponding toeach of said light receiving surfaces; means for producing a differencesignal of said output signals of said photosensor; means for producing asum signal of said output signals of said photosensor; a normalizationprocessing means including an A/D converter having an input forreceiving the difference signal of said output signals of saidphotosensor as an analog input signal and the sum signal of said outputsignals of said photosensor as a reference input signal and having anoutput which is an offset binary type digital signal, therebynormalizing said difference signal by said sum signal; and a positioningactuator controlled on the basis of said digital signal outputted fromsaid normalization processing means.
 2. An optical data medium drivingapparatus and a servo circuit as set forth in claim 1, wherein said A/Dconverter has two reference input terminals which are given said sumsignal as a positive reference input signal and a signal of invertedpolarity of said sum signal as a negative reference input signal,thereby outputting said digital signal.
 3. An optical data mediumdriving apparatus and a servo control as set forth in claim 1, whereinsaid A/D converter has one reference input terminal and is given saiddifference signal added with 1/2 of said sum signal as an analog inputsignal and said sum signal as said reference input signal, therebyoutputting said digital signal.
 4. An apparatus for processing signalsfrom a detector, the detector having at least first and second areas forgenerating electrical signals upon receiving a physical quantity,comprising:first operation means for deriving out the sum of theelectrical signals from at least the first and second areas of thedetector to generate a first operation signal, the first operationsignal having a predetermined amplitude; second operation means forderiving out the difference between the electrical signals from at leastthe first and second areas of the detector to generate a secondoperation signal; and converting means for converting the secondoperation signal generated by the second operation means into digitaldata in accordance with a conversion gain determined by the amplitude ofthe first operation signal generated by the first operation means.